Pristine, individual single-walled carbon nanotubes (SWNTs) have excellent electrical properties that exceed those of the semiconductors and metals currently used in microchip manufacturing. Depending on chirality and diameter, individual SWNTs may be semiconductive (s-SWNT) or metallic (m-SWNT). For s-SWNTs, the electron mobility is orders of magnitude greater than that for Si and GaAs.1 While in m-SWNTs, the mean free path for an electron can exceed 2 μm, making them prime candidates for use as electrical interconnects. Additionally, since conduction occurs via an extended π bonding network, they are not susceptible to electromigration, which results from the movement of metal nuclei in response to momentum transfer from electrons during current flow. This is an increasingly significant failure mechanism as device structures decrease in size.2 Therefore, both varieties of SWNTs have great potential in many microelectronics applications.
However, significant challenges remain for developing manufacturable electronic materials that make use of an individual SWNT as the active component, as one of the most notable characteristics of SWNTs is their polydispersity: for bulk growth processes, ⅓rd are m-SWNTs, while the other ⅔rd are s-SWNTs. Approaches to dealing with this problem include attempts at selective growth of s-SWNTs,3-5 or post growth solution processing to remove m-SWNTs.6-9 However, for s-SWNTs, the band gap varies with diameter and chirality from near 0 to ˜1.8 eV. Therefore, even after the separation of SWNTs based on their type of electrical conductivity, widely varying band gaps remain in the semiconductive portion, causing semiconductor device structures formed from individual SWNTs to be highly irreproducible. Additionally, the current drive through an individual SWNT is limited to the nA range, while higher current drives are needed by modern electronic devices. Further, device structures based on individual SWNTs will require significant advances in the ability to control the length, orientation, and location of SWNTs during their growth or deposition.
Therefore, 2-D SWNT networks represent a potential route to the widespread use of SWNTs. In a 2-D array, the nanotube density and alignment largely dictate performance. Also, multiple SWNTs connected in parallel provide orders of magnitude more current than an individual SWNT. Further, unlike Si-based electronic materials, SWNT networks have great potential in transparent, lightweight, and flexible electronic materials, especially as new aqueous suspension-based deposition methods are developed for the polymer substrates used in these applications.
A drawback to the use of SWNT networks is their greatly reduced performance, relative to that observed for single-SWNT systems. Several factors contribute to this reduced performance. First, inter-device precision is low in field-effect transistors (FETs) based on SWNT networks in part because changes in the Schottky barrier height between s-SWNTs in direct contact with the metal source and drain electrodes dictate much of the response to the gate voltage (Vg),10-12 leaving the semiconductive channel largely unaffected. Also, the OFF-state current of SWNT network-based FETs is limited by the presence of the metallic pathways provided by m-SWNTs and small band gap s-SWNTs, since they are largely unaffected by Vg. These effects combine to increase the OFF-state source/drain leakage currents in SWNT-based devices, greatly reducing their energy efficiency. Second, due to the inter-SWNT tunnel junctions that must be traversed in a network, their electron mobility decreases up to three orders of magnitude, relative to that for individual nanotubes.1,13 Also, the poor attractive forces between metals and the π bonding network in nanotubes results in non-ohmic contacts, increasing contact R and thereby reducing the level of ON-state current efficiency that can be achieved at a given source/drain voltage, reducing the ON-state/OFF-state current ratio (Ion/Ioff).